Research On Improvement Of CASS Treatment Process In Urban Wastewater And Enhanced Removal Of Pollutants

The removal of nitrogen-containing pollutants has always been a problem in sewage treatment.The traditional CASS treatment process is widely used in sewage treatment plants,but its general nitrogen treatment effect is generally poor.In this paper,the traditional CASS process is improved by adding nitrate liquid reflux and polyurethane sponge filler,respectively.The effects of operating parameters on the enhanced removal of total nitrogen,such as cycle period,aeration time,drainage ratio,and internal nitrification liquid reflux ratio,were studied.And the nitrogen removal mechanism of two improved CASS processes was investigated from net nitrogen conversion rate,microbial community structure and process nitrogen conversion routes.The results showed that the nitrification liquid reflux(referred to as CASS reflux process)and adding polyurethane sponge filler(referred to as CASS composite biofilm process)both had the ability to enhance the total nitrogen removal.Compared with the traditional CASS process,the TN removal efficiencies of the CASS reflux process and CASS composite biofilm process were increased by10-15% and 15-37%,respectively.At a circulation period of 24 h,and the initial TN concentration of 50mg/L,TN concentration of effluent in traditional CASS process was 20.77 mg/L,while decreased as low as 14.78 mg/L and 4.65 mg/L in CASS reflux and CASS composite biofilm processes,respectively.The degradation efficiencies of COD and NH3-N were the fastest among the three processes in a single cycle,and denitrification was the strongest in the early stage of water inflow.Compared with the traditional CASS process,the effects of TN decline in CASS reflux and CASS composite biofilm processes were faster.The cycle period,aeration time,drainage ratio,nitrate reflux ratio,etc are required to consider in the two improved processes.The study found that the longer the cycle period,the better the treatment effect of the two improved processes,and treatment effect of the CASS composite biofilm process was the best.The aeration time was adjusted to 4,5 and 6 h,which had little effects on t TN removal in the two improved processes.The ammonia nitrogen removal effect of the two improved processes increased with the increase of the aeration time.The drainage ratios were set to be 30%,50%,and 90%,and it was found that a smaller drainage ratio was beneficial to ammonia nitrogen oxidation and improved the removal capacity of total nitrogen in the two improved processes.Adjusting nitrate liquid reflux ratio to 20,35,and 50% in CASS reflux process,the studies found that the lowest ammonia nitrogen concentration was realized at 35%,and the TN of effluent was less than 15 mg/L at50%.Under the optimal operating conditions,the two CASS improved processes can exert the best treatment effect and strengthen on the TN removal.In heterotrophic nitrification and autotrophic nitrification,the nitrification of microorganisms in the three processes is mainly heterotrophic nitrification;and in bacterial nitrification and fungal nitrification,bacterial nitrification accounts for the main part.When the cycle period was set at 8 and 12 h,the nitrification rates of the three processes were higher,and the net nitrification rate of the CASS traditional process was the lowest.The nitrification rate of polyurethane sponge filler in CASS composite biofilm process was generally higher than that of suspended activated sludge.The dominant bacteria in the CASS reactor were mainly consisted by bacteria with the function of degrading organic matter,such as Saccharibacteria,Propioniciclava and so on.Nitrospira,a common nitrifying bacterium,was detected in the three reactors.The proportions in the traditional CASS process,CASS reflux process,and CASS composite biofilm process were 0.21%,0.08%,and 0.71%,respectively.The denitrifying bacteria such as Thauera,Rhodobacter,Phaeodactylibacter in the CASS composite biofilm process had the highest content,and were concentrated in a large amount of polyurethane fillers.It can be seen that the CASS composite biofilm system is more conducive to the growth of denitrifying functional bacteria.